MICROSCOPIC OBJECTS. 133 



If the subject is to be mounted as a transparent 

 object, as soon as soft it must be laid on a slip of glass 

 and spread out ; it is then to be covered with another 

 sHp of glass, and the two bound together by thin twine. 

 In this condition the object must remain until quite dry, 

 when the slips of glass are to be separated. It is now 

 ready for mounting, the instructions for which will be 

 given in a subsequent yjage. 



On Mounting Microscopic Objects in Canada Balsam. — 

 We are indebted to Mr. Pritchard for the suggestion of 

 this superb mode of embalming. In the * Microscopic 

 Cabinet ' is given the first account of mounting objects 

 in fluid, which afterwards hardened and rendered them 

 permanent. This fluid is a thick solution of gum arable, 

 which when indurated is found to be very durable if kept 

 in a dry place, but with damp is hable to mildew. This 

 contrivance led to the emplojinent of other media that 

 should be free from this defect. Varnishes, such as the 

 mastic, copal, &c., were severally tried, and each was 

 found to possess some advantages ; but the substance 

 which is found to surpass all others is Canada balsam. 

 Its merits are, that when once hard the preparation is 

 permanent ; is not affected by damp ; the slide can be 

 readily cleansed without injury ; it remains clear and 

 transparent ; and, lastly, its refractive power is so great 

 that bodies mounted in it are rendered sufficiently per- 

 vious to light to show their internal structure, which 

 without it are too opaque to transmit a single ray. 



So numerous are the objects that derive additional 



